1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit comprising thin film transistors (hereinafter referred to as TFTs). In particular, the present invention relates to a technique for manufacturing a TFT in a substrate having a low heat resistance such as a plastic substrate. Note that the semiconductor devices in this specification indicate devices in general which function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic equipment in which those units are mounted are included in the category of the semiconductor devices.
2. Description of the Related Art
Since a thin film transistor (hereinafter referred to as TFT) can be formed on a transparent substrate, development for applications to an active matrix liquid crystal display (hereinafter referred to as an AM-LCD) is in active progress. Since high mobility is obtained in the case of a TFT using a semiconductor film having a crystal structure (hereinafter referred to as crystalline semiconductor film), functional circuits can be integrated on the same substrate to realize high resolution image display. In particular, with respect to an active matrix display device represented by a liquid crystal display device, a technique for forming not only a pixel portion but also a driver circuit by using TFTs is under development.
With respect to such a display device, ones of varying sizes are manufactured. In particular, applications to a mobile electronic device are noted. In the case of the mobile electronic device, it is said that weight reduction of individual parts is required and the display device is no different. Until now, a glass plate having a thickness of about 1 mm is used as a substrate composing the display device. However, a method of using a plastic substrate is examined for the weight reduction thereof.
However, since a process temperature of the TFT using the crystalline semiconductor film is about 400xc2x0 C., it is impossible to apply this technique to the plastic substrate as it stands. When the TFT is formed on the plastic substrate, it is necessary to form the crystalline semiconductor film, a gate insulating film, and the like at about 300xc2x0 C. or lower. A film formation technique using a sputtering method is one of the techniques capable of forming not only a conductive film but also an insulating film and a semiconductor film at a relatively low temperature. An attempt has already been made, in which the gate insulating film of the TFT is formed by a sputtering method. However, preferable characteristics cannot be necessarily obtained.
An index indicating TFT characteristics includes a subthreshold coefficient (S value). This is obtained based on a relationship between a drain current flowed in a weak inverse state when a voltage close to a threshold voltage or a lower voltage is applied to the gate electrode and the gate voltage. Generally, the smaller the subthreshold coefficient is, the more possible a switching operation at high speed and low consumption power becomes. The TFT characteristics in the case where a gate insulating film is formed by a sputtering method is reported by, for example, Serikawa (Japan, J. Appl. Phys. Vol. 39 (2000), pp. L393-395). According to this report, when an SiO2 target is used and the gate insulating film is formed by using a sputtering gas of oxygen (30%) and argon (70%), the S value is 2.5 V/dec in an n-channel TFT and 1.8 eV/dec is obtained in a p-channel TFT.
When a switching characteristic required in the case where a TFT is applied to a display device and the like is considered, this S value is not necessarily enough one. It is considered that there are various factors for deteriorating the TFT characteristic in the case where the gate insulating film is formed by a sputtering method, and that the control of concentration of hydrogen included in the insulting film is a most important factor.
Also, since a large number of traps are present in a crystalline silicon layer or in an interface between the crystalline silicon layer and a gate insulating film, there is a problem in that a threshold voltage (Vth) of a TFT is increased or high mobility is not obtained. Thus, it is required that hydrogen be diffused into the crystalline silicon layer or the interface between the crystalline silicon layer and the gate insulating film to terminate the traps so that film property of the crystalline silicon film is improved.
Until now, there is a case where a TFT is manufactured by a film formation method of controlling the amount of hydrogen added into a film. According to, for example, Japanese Patent Application Laid-open No. Hei 10-0845085, an interlayer film made from a silicon nitride film including hydrogen or a silicon oxynitride film including hydrogen is formed over a TFT and hydrogen is diffused from the silicon nitride film including hydrogen or the silicon oxynitride film including hydrogen to the TFT. Thus, a transistor characteristic can be made stable by hydrogenation processing at a lower substrate temperature. Note that, when the silicon nitride film or the silicon oxynitride film including hydrogen is formed by a film formation method in which a substrate processing temperature exceeds 400xc2x0 C. it is difficult to apply such a method to a plastic substrate.
The present invention is to provide a technique for solving such a problem, and an object thereof is therefore to form an insulating film by sputtering on a plastic substrate which is low in upper limit of a process temperature to thereby improve the characteristics of a TFT.
The present invention is characterized in that a sputtering method is used and a film formation condition is divided into a first stage and a second stage in the case where the gate insulating film of a TFT is formed. In the first stage, the gate insulating film is formed so as to include hydrogen therein at a concentration of 0.4 atomic % to 1.6 atomic %. For example, a method of performing sputtering with a silicon target using argon, N2O, and hydrogen as sputtering gases is employed. In the second stage, the gate insulating film is formed so as to include hydrogen therein at a concentration of 0.2 atomic % or lower. For example, a method of performing sputtering with an SiO2 target using argon and oxygen as sputtering gases is employed.
An object of the first stage is to include hydrogen in the gate insulating film and an object of the second stage is to form a dense film in which the amount of hydrogen to be included is less and to prevent hydrogen included in the gate insulating film in the first state from being released to the outside. Thus, the gate insulating film of the present invention is formed in a two-layer structure.
When hydrogen is included at the above concentration in an interface between the gate insulating film and a semiconductor layer and its vicinity by a first film formation method, a defect density in the interface and the film can be reduced. Also, internal stress is relaxed (decreased) to reduce stress applied to the semiconductor layer and the interface and thus not only an S value but also a shift in a threshold voltage can be suppressed. Further, a part of hydrogen included in a region of the gate insulating film formed in the first stage can be diffused to contribute to hydrogenation of the semiconductor layer. The gate insulating film formed in the second stage is a relatively dense film. Thus, when hydrogen produced in the first stage is contained to prevent diffusion thereof to the outside, it is possible to keep the effect of hydrogen.
A substrate temperature at the formation of the gate insulating film is set to be 300xc2x0 C. in both the first stage and the second stage. As a result the gate insulating film having the two-layer structure can be formed for a plastic substrate.
As described above, a method of manufacturing a semiconductor device according to the present invention is characterized by including the steps of: forming a semiconductor layer on an insulating surface; forming a gate insulating film on the semiconductor layer; and forming a conductive layer on the gate insulating film, in that the step of forming the gate insulating film is performed by a sputtering method using Si or SiO2 as a target and has a first stage for forming a first insulating film including mainly Si, oxygen, and nitrogen and further including hydrogen at a concentration of 0.4 atomic % to 1.6 atomic % and a second stage for forming a second insulating film including mainly Si and oxygen and further including hydrogen at a concentration of 0.2 atomic % or lower.
Also, since the present invention is characterized in that film formation is performed at a low temperature it is very suitable for a plastic substrate, while it can be also applied to a glass substrate and a quartz substrate. Even when it is applied to a glass substrate and a quartz substrate, effects such as cost reduction and improvements of throughput due to a decrease in a process temperature are obtained.
Note that there are an SIMS analysis and a hydrogen forward scattering (HFS) analysis as typical methods of measuring a hydrogen concentration in a film. The HFS analysis is a method of irradiating an He++ ion to a sample and measuring energy of a scattered He++ ion to determine a sample composition from a loss of the energy. The reason why the hydrogen concentration in the film is measured by the HFS analysis is as follows. That is, in the case where the concentration of hydrogen becomes about several % as the film of the present invention, when it is measured by the SIMS analysis, a measurement error is increased due to a matrix effect. Thus, the present inventor determined that the HFS analysis is reasonable. Also, in the case of the HFS analysis, an He++ ion as a probe is irradiated to a sample in a range of xcfx86 of 5 mm to 10 mm. When xcfx86 is in the range of 5 mm to 10 mm and a film thickness is 10 nm or more, the measurement is possible. When an opening and the like are present in the range of xcfx86 of 5 mm to 10 mm in a film to be analyzed by the HFS analysis to produce an uneven portion, although the influence of a base is caused the measurement is possible. Thus, when a layer located on a film to be analyzed is peeled to be exposed in a completed semiconductor device, the HFS analysis is possible.
According to the HFS analysis performed in this specification, a result such that hydrogen is included at 0.2 atomic % or lower in the second stage is obtained. With respect this result, there is also a possibility that the concentration of hydrogen is 0 atomic % because of an analysis error.